In the bone tissue, collagen and hydroxyapatite interact on a nano level, and then hydroxyapatite is aligned along the collagen fibers to obtain assembly thereof. Further, a hierarchic structure is formed by the assembly. The bone tissue exhibits excellent mechanical characteristics through the hierarchic structure and an interaction between collagen fibers.
In the case of loss of bone tissue, an autologous bone graft using an iliac bone, fibula or the like of a patient is mainly performed as a treatment thereof. However, in the autologous bone graft, a secondary operation is required, and thus it is a high burden on the patients. Further, an infection sometimes occurs in the exenteration region of the secondary operation. Furthermore, the amount of the autologous bone to be collected is limited. Thus, an attempt to use an artificial bone instead of the autologous bone, or an attempt to supply an artificial bone (i.e. filling material) to a deficient part of the autologous bone, has been made.
Hitherto, as the artificial bone, ceramic-based artificial bone made of calcium phosphate with an osteoconduction is used. However, a postoperative break and a poor handling due to a brittleness which are typical characteristics of ceramic, have been pointed out in the clinical site. Further, the artificial bone requires specific physiological properties, such as a biocompatibility and an osteoconductive property. The autologous bone metabolizes and thus alternates between resorption and ossification. However, the artificial bone made of apatite can hardly dissolve in a body, and thus almost permanently remains therein. The artificial bone used in bone regeneration requires a property for fusing to bone tissue and promoting bone regeneration, in addition to bone-seeking. That is, the artificial bone requires the osteoconductive property and a bioactivity, in order to be absorbed gradually after applying to the body, become incorporated into a bone regeneration cycle (bone remodeling), and be replaced with the individual's bone tissue.
In view of the above points, a calcium phosphate-collagen composite wherein collagen is added to calcium phosphate, is desired as the artificial bone for bone regeneration. The calcium phosphate-collagen composite has osteoconduction and exhibits a flexibility which is deficient in the conventional ceramic. Therefore, the calcium phosphate-collagen composite is studied for practical use, as a scaffold material for bone regeneration. However, the calcium phosphate-collagen composite is too soft, compared to the conventional material made of apatite, and thus it easily becomes deformed by a lower load. Therefore, a problem emerges where the calcium phosphate-collagen composite cannot be applied to a high-load region as a bone regeneration template.
The development of the calcium phosphate-collagen composite is oriented in two directions. One direction is to attach weight to the osteoconductive property as the scaffold material for bone regeneration, and the other direction is to attach weight to a mechanical strength as the template of bone regeneration. As an example of the former calcium phosphate-collagen composite, the patent literature 1 discloses that a dried apatite-collagen composite fiber is gamma-irradiated to obtain a porous material wherein a half-life of strength is shortly adjusted. On the other hand, as an example of the latter calcium phosphate-collagen composite, the patent literature 2 discloses that a concentration of collagen fibers is increased and thereby the density thereof may rise to 130 to 600 mg/cm3 in order to obtain a calcium phosphate-collagen fiber composite having excellent mechanical properties such as a high strength and a high elasticity close to those of autologous bone in the body.
However, a calcium phosphate-collagen composite having an excellent mechanical property has not been developed. That is, a calcium phosphate-collagen composite which can be rapidly replaced with the patient's own bone tissue via bone remodeling and can be used in a high-load region as the scaffold material for bone regeneration, has not been developed.